Hypoglycaemia and hyperinsulinaemia in response to raised free-fatty-acid levels

The biochemical alterations underlying post-burn hypermetabolism

A severe burn can trigger a hypermetabolic state which lasts for years following the injury, to the detriment of the patient. The drastic increase in metabolic demands during this phase renders it difficult to meet the body's nutritional requirements, thus increasing muscle, bone and adipose catabolism and predisposing the patient to a host of disorders such as multi-organ dysfunction and sepsis, or even death. Despite advances in burn care over the last 50 years, due to the multifactorial nature of the hypermetabolic phenomenon it is difficult if not impossible to precisely identify and pharmacologically modulate the biological mediators contributing to this substantial metabolic derangement. Here, we discuss biomarkers and molecules which play a role in the induction and mediation of the hypercatabolic condition post-thermal injury. Furthermore, this thorough review covers the development of the factors released after burns, how they induce cellular and metabolic dysfunction, and how these factors can be targeted for therapeutic interventions to restore a more physiological metabolic phenotype after severe thermal injuries. This article is part of a Special Issue entitled: Immune and Metabolic Alterations in Trauma and Sepsis edited by Dr. Raghavan Raju.

Key Questions for Translation of FFA Receptors: From Pharmacology to Medicines

The identification of fatty acids as ligands for the G-protein coupled free fatty acid (FFA) receptor family over 10 years ago led to intensive chemistry efforts to find small-molecule ligands for this class of receptors. Identification of potent, selective modulators of the FFA receptors and their utility in medicine has proven challenging, in part due to their complex pharmacology. Nevertheless, ligands have been identified that are sufficient for exploring the therapeutic potential of this class of receptors in rodents and, in the case of FFA1, FFA2, FFA4, and GPR84, also in humans. Expression profiling, the phenotyping of FFA receptor knockout mice, and the results of studies exploring the effects of these ligands in rodents have uncovered a number of indications where engagement of one or a combination of FFA receptors might provide some clinical benefit in areas including diabetes, inflammatory bowel syndrome, Alzheimer's, pain, and cancer. In this chapter, we will review the clinical potential of modulating FFA receptors based on preclinical and in some cases clinical studies with synthetic ligands. In particular, key aspects and challenges associated with small-molecule ligand identification and FFA receptor pharmacology will be addressed with a view of the hurdles that need to be overcome to fully understand the potential of the receptors as therapeutic targets.

Insulin secretion is increased in non-diabetic subjects with fasting hypertriglyceridaemia

BACKGROUND

The elevation of triglycerides is strongly linked with insulin resistance, but it has not been evaluated in relationship to insulin secretion. The aim of this study was to determine whether hypertriglyceridaemia is associated with abnormal insulin secretion.

METHODS

A cross-sectional study was carried out. Eligible subjects, apparently healthy men and non-pregnant women aged 20-65 years were recruited. According to the triglyceride levels, subjects were allocated in the groups with hypertriglyceridaemia and normotriglyceridaemia. Hypertriglyceridaemia was defined by serum triglyceride levels ≥150 mg/dL. Insulin secretion was evaluated by the first phase of insulin secretion (1st PIS) and the second phase of insulin secretion (2nd PIS). A regression linear analysis was performed to evaluate the association between hypertriglyceridaemia (independent variable) and the first and second phase insulin secretion (dependent variables).

RESULTS

A total of 247 apparently healthy subjects were enrolled; 113 (45.7%) with hypertriglyceridaemia and 134 (54.3%) in the control group. The simple regression linear analysis showed a significant association between hypertriglyceridaemia and the 1st PIS [B = 207.0; 95% confidence interval (CI) 33.5-380.5, p = 0.02] and the 2nd PIS (B = 48.7; 95% CI 9.2-88.2, p = 0.01). A multiple regression linear analysis adjusted by age, sex, body mass index and waist circumference was performed showing that fasting hypertriglyceridaemia remained significantly associated with the 1st PIS (B = 184.3; 95% CI 13.0-355.7, p = 0.03) and the 2nd PIS (B = 43.1; 95% CI 4.2-81.9, p = 0.03).

CONCLUSIONS

The results of this study show that hypertriglyceridaemia is associated with the increase of the 1st PIS and the 2nd PIS in apparently healthy subjects.

Fatty acid binding receptors and their physiological role in type 2 diabetes

G-protein-coupled receptors (GPCRs) respond to various physiological ligands such as photons, ions, and small molecules that include amines, fatty acids, and amino acids to peptides, proteins and steroids. Therefore, this family of proteins represents an attractive target for biopharmaceutical research. The physiological role of fatty acids and other lipid molecules as important signal mediators is well studied in various metabolic pathways. Acute administration of free fatty acids (FFAs) stimulates insulin release. Conversely, chronic exposure to high levels of free fatty acids leads to impairment of beta cell function and lipotoxicity. However, the receptors through which these fatty acids and lipids act were unknown, until the identification of fatty acid binding receptors: GPR40, GPR41, GPR43, and GPR119. Based on their tissue-expression profile, and pharmacologic analysis, the fatty acid binding receptors along with lipid binding receptor GPR119 are linked to diabetes and obesity. They play a critical role in the metabolic regulation of insulin release and glucose homeostasis. In this review, the mechanism of receptor activation, pharmacology, and the physiological functions of the fatty acid binding receptors will be discussed.

Effects of aliphatic dioic acids and glycerol-1,2,3-tris(dodecanedioate) on D-glucose-stimulated insulin release in rat pancreatic islets

Aliphatic dioic acids have been proposed as alternative nutrients in selected clinical situations. In this study, their possible insulinotropic action was investigated in isolated rat pancreatic islets prepared from fed rats. Azelaic acid, sebacic acid and tridecanedioic acids, when tested at a 10·0 mM CONCENTRATION, WERE FOUND TO AUGMENT INSULIN RELEASE EVOKED BY d-glucose (7·0 mm) in the pancreatic islets. Likewise, glycerol-1,2,3-tris(dodecanoedioate), when used at concentrations close to 1·0 mm, increased the secretory response to the hexose. It is speculated that these findings may extend to insulin-producing cells, the knowledge that aliphatic dioic acids or their esters may act as energy substrates, e.g. in parenteral nutrition.

Oleic acid interacts with GPR40 to induce Ca2+ signaling in rat islet beta-cells: mediation by PLC and L-type Ca2+ channel and link to insulin release

It has long been thought that long-chain free fatty acids (FFAs) stimulate insulin secretion via mechanisms involving their metabolism in pancreatic beta-cells. Recently, it was reported that FFAs function as endogenous ligands for GPR40, a G protein-coupled receptor, to amplify glucose-stimulated insulin secretion in an insulinoma cell line and rat islets. However, signal transduction mechanisms for GPR40 in beta-cells are little known. The present study was aimed at elucidating GPR40-linked Ca(2+) signaling mechanisms in rat pancreatic beta-cells. We employed oleic acid (OA), an FFA that has a high affinity for the rat GPR40, and examined its effect on cytosolic Ca(2+) concentration ([Ca(2+)](i)) in single beta-cells by fura 2 fluorescence imaging. OA at 1-10 microM concentration-dependently increased [Ca(2+)](i) in the presence of 5.6, 8.3, and 11.2 mM, but not 2.8 mM, glucose. OA-induced [Ca(2+)](i) increases at 11.2 mM glucose were inhibited in beta-cells transfected with small interfering RNA targeted to rat GPR40 mRNA. OA-induced [Ca(2+)](i) increases were also inhibited by phospholipase C (PLC) inhibitors, U73122 and neomycin, Ca(2+)-free conditions, and an L-type Ca(2+) channel blocker, nitrendipine. Furthermore, OA increased insulin release from isolated islets at 8.3 mM glucose, and it was markedly attenuated by PLC and L-type Ca(2+) channel inhibitors. These results demonstrate that OA interacts with GPR40 to increase [Ca(2+)](i) via PLC- and L-type Ca(2+) channel-mediated pathway in rat islet beta-cells, which may be link to insulin release.

Insulin secretion in lipodystrophic HIV-infected patients is associated with high levels of nonglucose secretagogues and insulin resistance of beta-cells

We examined whether plasma concentrations of nonglucose insulin secretagogues are associated with prehepatic insulin secretion rates (ISR) in nondiabetic, insulin-resistant, human immunodeficiency virus (HIV)-infected, lipodystrophic patients (LIPO). Additionally, the negative feedback of insulin on ISR was evaluated. ISR were estimated by deconvolution of plasma C-peptide concentrations during fasting (basal) and during the last 30 min of a 120-min euglycemic insulin clamp (40 mU.m(-2).min(-1)). Eighteen normoglycemic LIPO were compared with 25 normoglycemic HIV-infected patients without lipodystrophy (controls). Thirty minutes before start of the clamp, a bolus of glucose was injected intravenously to stimulate endogenous insulin secretion. Insulin sensitivity index (SiRd) was estimated from glucose tracer analysis. LIPO displayed increased basal ISR (69%), clamp ISR (114%), basal insulin (130%), and clamp insulin (32%), all P < or = 0.001, whereas SiRd was decreased (57%, P < 0.001). In LIPO, ISRbasal correlated significantly with basal insulin, alanine, and glucagon (all r > 0.65, P < 0.01), but not with glucose. In control subjects, ISR(basal) correlated significantly with insulin, glucagon, and glucose (all r > 0.41, P < 0.05), but not with alanine. In LIPO, ISRclamp correlated significantly with clamp free fatty acids (FFA), alanine, triglyceride, and glucagon (all r > 0.51, P < 0.05). In control subjects, ISRclamp correlated with clamp triglyceride (r = 0.45, P < 0.05). Paradoxically, in LIPO, ISRclamp correlated positively with clamp insulin (r = 0.68, P < 0.01), which suggests an absent negative feedback of insulin on ISR. Our data support evidence that lipodystrophic, nondiabetic, HIV-infected patients exhibit increased ISR, which can be partially explained by an impaired negative feedback of insulin on beta-cells and an increased stimulation of ISR by FFA, alanine, triglyceride, and glucagon.

The nicotinic acid receptor--a new mechanism for an old drug

CONTEXT

Non-esterified fatty acids in plasma originate from adipose tissue. Delivery of fatty acids to the liver provides the substrate for VLDL triglycerides. Insulin-sensitive organs, overburdened by high concentrations of non-esterified fatty acids, may develop resistance to insulin action. In addition, insulin secretion from pancreatic beta-cells may be impaired by long-standing elevation of concentrations of non-esterified fatty acid in plasma. Normally, such concentrations fluctuate over the day depending on the transient suppression of lipolysis from adipose tissue by insulin released after meals. Diurnal concentrations of non-esterified fatty acid are often elevated in obesity, in particular in male-pattern upper-body fat accumulation. Nicotinic acid is the only drug that primarily lowers concentrations of non-esterified fatty acids and thereby lowers VLDL triglycerides. Nicotinic acid, or its analogues, seems to alleviate insulin resistance in the short-term whereas, paradoxically, the long-term effect is often the opposite. Suppression of lipolysis by nicotinic acid gives rise to a prominent rebound and the degree to which this occurs might explain this paradox.

STARTING POINT

The exact cellular mechanism by which nicotinic acid exerts its antilipolytic effects has not been known until the recent discovery of a distinct G-protein coupled receptor. Nicotinic acid is a high affinity ligand, but the endogenous ligand is still unknown. Recently, Tina Rubic and colleagues (Biochem Pharmacol 2004; 67: 411-19) proposed a mechanism in which nicotinic acid stimulates cholesterol mobilisation from macrophages, thereby providing a potential link between regression of atherosclerosis and use of nicotinic acid.

WHERE NEXT

Research on signalling through the nicotinic acid receptor might give rise to novel and more effective methods to interfere with fatty-acid metabolism, with insulin resistance, hyperlipidaemia, and atherosclerosis as target diseases.

Acute effects of meal fatty acid composition on insulin sensitivity in healthy post-menopausal women

Postprandial plasma insulin concentrations after a single high-fat meal may be modified by the presence of specific fatty acids although the effects of sequential meal ingestion are unknown. The aim of the present study was to examine the effects of altering the fatty acid composition in a single mixed fat-carbohydrate meal on glucose metabolism and insulin sensitivity of a second meal eaten 5 h later. Insulin sensitivity was assessed using a minimal model approach. Ten healthy post-menopausal women underwent four two-meal studies in random order. A high-fat breakfast (40 g fat) where the fatty acid composition was predominantly saturated fatty acids (SFA), n-6 polyunsaturated fatty acids (PUFA), long-chain n-3 PUFA or monounsaturated fatty acids (MUFA) was followed 5 h later by a low-fat, high-carbohydrate lunch (5.7 g fat), which was identical in all four studies. The plasma insulin response was significantly higher following the SFA meal than the other meals after both breakfast and lunch (P<0.006) although there was no effect of breakfast fatty acid composition on plasma glucose concentrations. Postprandial insulin sensitivity (SI(Oral)) was assessed for 180 min after each meal. SI(Oral) was significantly lower after lunch than after breakfast for all four test meals (P=0.019) following the same rank order (SFA < n-6 PUFA < n-3 PUFA < MUFA) for each meal. The present study demonstrates that a single meal rich in SFA reduces postprandial insulin sensitivity with 'carry-over' effects for the next meal.

Insulin resistance directly correlates with increased saturated fatty acids in skeletal muscle triglycerides

A close relationship between elevated plasma free fatty acid (FFA) levels and insulin resistance is commonly reported in obese subjects. The aim of the present study was to evaluate the role of intramuscular triglyceride (mTG) and FFA levels in insulin sensitivity in 30 nondiabetic normal-weight or obese subjects (18 with body mass index [BMI] = 21.8 +/- 3.3 kg/m2 and 12 with BMI = 34.6 +/- 2.7 kg/m2) who underwent minor abdominal surgery. Body composition was estimated by isotopic dilution, substrate oxidation by indirect calorimetry, and whole-body glucose uptake by euglycemic-hyperinsulinemic clamp (EHC). Glucose uptake (M) value negatively correlated with the MTG level (R2 = -.56, P < .0001), which was increased in obese patients (11.6 +/- 2.2 v 6.2 +/- 1.4 micromol/g wet weight muscle tissue, P < .0001). The TG fatty acid profile was significantly different in the 2 groups: an increased concentration of saturated fat was present in obese patients (unsaturated to saturated ratio, 1.89 +/- 0.40 v2.19 +/- 0.07, P < .0001). Stepwise linear regression analysis of total mTGs and palmitic and oleic fractions on the M value showed that only TGs and palmitic acid were significantly related to glucose uptake (R2 = .66, P < .0001). Furthermore, among the other anthropometric variables, only the BMI was significantly correlated with MTGs (R2 = .71, P < .0001). In conclusion, not only the MTG concentration but also the FFA pattern seems to affect insulin-mediated glucose uptake. A pivotal role might be played by a high saturated fatty acid content in the TGs.

Relative hypoglycemia and hyperinsulinemia in mice with heterozygous lipoprotein lipase (LPL) deficiency. Islet LPL regulates insulin secretion

Lipoprotein lipase (LPL) provides tissues with fatty acids, which have complex effects on glucose utilization and insulin secretion. To determine if LPL has direct effects on glucose metabolism, we studied mice with heterozygous LPL deficiency (LPL+/-). LPL+/- mice had mean fasting glucose values that were up to 39 mg/dl lower than LPL+/+ littermates. Despite having lower glucose levels, LPL+/- mice had fasting insulin levels that were twice those of +/+ mice. Hyperinsulinemic clamp experiments showed no effect of genotype on basal or insulin-stimulated glucose utilization. LPL message was detected in mouse islets, INS-1 cells (a rat insulinoma cell line), and human islets. LPL enzyme activity was detected in the media from both mouse and human islets incubated in vitro. In mice, +/- islets expressed half the enzyme activity of +/+ islets. Islets isolated from +/+ mice secreted less insulin in vitro than +/- and -/- islets, suggesting that LPL suppresses insulin secretion. To test this notion directly, LPL enzyme activity was manipulated in INS-1 cells. INS-1 cells treated with an adeno-associated virus expressing human LPL had more LPL enzyme activity and secreted less insulin than adeno-associated virus-beta-galactosidase-treated cells. INS-1 cells transfected with an antisense LPL oligonucleotide had less LPL enzyme activity and secreted more insulin than cells transfected with a control oligonucleotide. These data suggest that islet LPL is a novel regulator of insulin secretion. They further suggest that genetically determined levels of LPL play a role in establishing glucose levels in mice.

A fatty acid- dependent step is critically important for both glucose- and non-glucose-stimulated insulin secretion

Lowering of the plasma FFA level in intact fasted rats by infusion of nicotinic acid (NA) caused essentially complete ablation of insulin secretion (IS) in response to a subsequent intravenous bolus of arginine, leucine, or glibenclamide (as previously found using glucose as the beta-cell stimulus). However, in all cases, IS became supranormal when a high FFA level was maintained by co-infusion of lard oil plus heparin. Each of these secretagogues elicited little, if any, IS from the isolated, perfused "fasted" pancreas when tested simply on the background of 3 mM glucose, but all became extremely potent when 0.5 mM palmitate was also included in the medium. Similarly, IS from the perfused pancreas, in response to depolarizing concentrations of KCl, was markedly potentiated by palmitate. As was the case with intravenous glucose administration, fed animals produced an equally robust insulin response to glibenclamide regardless of whether their low basal FFA concentration was further reduced by NA. In the fasted state, arginine-induced glucagon secretion appeared to be independent of the prevailing FFA concentration. The findings establish that the essential role of circulating FFA for glucose-stimulated IS after food deprivation also applies in the case of nonglucose secretagogues. In addition, they imply that (i) a fatty acid-derived lipid moiety, which plays a pivotal role in IS, is lost from the pancreatic beta-cell during fasting; (ii) in the fasted state, the elevated level of plasma FFA compensates for this deficit; and (iii) the lipid factor acts at a late step in the insulin secretory pathway that is common to the action of a wide variety of secretagogues.

Tissue triglycerides, insulin resistance, and insulin production: implications for hyperinsulinemia of obesity

Obesity is associated with both insulin resistance and hyperinsulinemia. Initially hyperinsulinemia compensates for the insulin resistance and thereby maintains normal glucose homeostasis. Obesity is also associated with increased tissue triglyceride (TG) content. To determine whether both insulin resistance and hyperinsulinemia might be secondary to increased tissue TG, we studied correlations between TG content of skeletal muscle, liver, and pancreas and plasma insulin, plasma [insulin] x [glucose], and beta-cell function in four rat models with widely varying fat content: obese Zucker diabetic fatty rats, free-feeding lean Wistar rats, hyperleptinemic Wistar rats with profound tissue lipopenia, and rats pair fed to hyperleptinemics. Correlation coefficients >0.9 (P < 0.05) were obtained among TG of skeletal muscle, liver, and pancreas and among plasma insulin, [insulin] x [glucose] product, and beta-cell function as gauged by basal, glucose-stimulated, and arginine-stimulated insulin secretion by the isolated perfused pancreas. Although these correlations cannot prove cause and effect, they are consistent with the hypothesis that the TG content of tissues sets the level of both insulin resistance and insulin production.

The insulinotropic potency of fatty acids is influenced profoundly by their chain length and degree of saturation

Lowering of the elevated plasma FFA concentration in 18- 24-h fasted rats with nicotinic acid (NA) caused complete ablation of subsequent glucose-stimulated insulin secretion (GSIS). Although the effect of NA was reversed when the fasting level of total FFA was maintained by coinfusion of soybean oil or lard oil (plus heparin), the more saturated animal fat proved to be far more potent in enhancing GSIS. We therefore examined the influence of individual fatty acids on insulin secretion in the perfused rat pancreas. When present in the perfusion fluid at 0.5 mM (in the context of 1% albumin), the fold stimulation of insulin release from the fasted pancreas in response to 12.5 mM glucose was as follows: octanoate (C8:0), 3.4; linoleate (C18:2 cis/cis), 5.3; oleate (C18:1 cis), 9.4; palmitate (C16:0), 16. 2; and stearate (C18:0), 21.0. The equivalent value for palmitoleate (C16:1 cis) was 3.1. A cis--> trans switch of the double bond in the C16:1 and C18:1 fatty acids had only a modest, if any, impact on their potency. A similar profile emerged with regard to basal insulin secretion (3 mM glucose). When a subset of these fatty acids was tested in pancreases from fed animals, the same rank order of effectiveness at both basal and stimulatory levels of glucose was seen. The findings reaffirm the essentiality of an elevated plasma FFA concentration for GSIS in the fasted rat. They also show, however, that the insulinotropic effect of individual fatty acids spans a remarkably broad range, increasing and decreasing dramatically with chain length and degree of unsaturation, respectively. Thus, for any given level of glucose, insulin secretion will be influenced greatly not only by the combined concentration of all circulating (unbound) FFA, but also by the makeup of this FFA pool. Both factors will likely be important considerations in understanding the complex interplay between the nature of dietary fat and whole body insulin, glucose, and lipid dynamics.

Essentiality of circulating fatty acids for glucose-stimulated insulin secretion in the fasted rat

We asked whether the well known starvation-induced impairment of glucose-stimulated insulin secretion (GSIS) seen in isolated rat pancreas preparations also applies in vivo. Accordingly, fed and 18-24-h-fasted rats were subjected to an intravenous glucose challenge followed by a hyperglycemic clamp protocol, during which the plasma-insulin concentration was measured. Surprisingly, the acute (5 min) insulin response was equally robust in the two groups. However, after infusion of the antilipolytic agent, nicotinic acid, to ensure low levels of plasma FFA before the glucose load, GSIS was essentially ablated in fasted rats, but unaffected in fed animals. Maintenance of a high plasma FFA concentration by coadministration of Intralipid plus heparin to nicotinic acid-treated rats (fed or fasted), or further elevation of the endogenous FFA level in nonnicotinic acid-treated fasted animals by infusion of etomoxir (to block hepatic fatty acid oxidation), resulted in supranormal GSIS. The in vivo findings were reproduced in studies with the perfused pancreas from fed and fasted rats in which GSIS was examined in the absence and presence of palmitate. The results establish that in the rat, the high circulating concentration of FFA that accompanies food deprivation is a sine qua non for efficient GSIS when a fast is terminated. They also serve to underscore the powerful interaction between glucose and fatty acids in normal beta cell function and raise the possibility that imbalances between the two fuels in vivo could have pathological consequences.

Defective fatty acid-mediated beta-cell compensation in Zucker diabetic fatty rats. Pathogenic implications for obesity-dependent diabetes

Although obesity is associated with insulin resistance, most obese humans and rodents remain normoglycemic because of compensatory hyperinsulinemia. This has been attributed to beta-cell hyperplasia and increased low Km glucose metabolism of islets. Since free fatty acids (FFA) can induce these same beta-cell changes in normal islets of Wistar rats and since plasma FFA are increased in obesity, FFA could be the signal from adipocytes that elicits beta-cell compensation sufficient to prevent diabetes. To determine if FFA-induced compensation is impaired in islets of rats with a diabetogenic mutation, the Zucker diabetic fatty (ZDF) rat, we cultured islets from 6-week-old obese (fa/fa) rats that had compensated for obesity and apparently normal islets from lean ZDF rats (fa/+) in 0, 1, or 2 mM FFA. Low Km glucose usage rose 2.5-fold in FFA-cultured control islets from age-matched Wistar rats, but failed to rise in either the precompensated islets of ZDF rats or in islets of lean ZDF rats. Bromodeoxyuridine incorporation increased 3.2-fold in Wistar islets but not in islets from obese or lean ZDF rats. Insulin secretion doubled in normal islets cultured in 2 mM FFA (p < 0.01) but increased only slightly in islets from lean ZDF rats (not significant) and declined in islets from obese ZDF rats (p < 0.05). We conclude that, unlike the islets of age-matched Wistar rats, islets of 6-week-old heterozygous and homozygous ZDF rats lack the capacity for FFA-induced enhancement of beta-cell function.

Hemodynamic, respiratory, and metabolic effects of medium-chain triglyceride-enriched lipid emulsions following valvular heart surgery

STUDY

A lipid emulsion containing 10 percent medium-chain triglycerides (MCT) and 10 percent long-chain triglycerides (LCT) was infused at a rate of 1 ml/kg/h (3.3 mg/kg/min) for 2 h, in 12 patients (2 males, 10 females; mean age, 54 +/- 3 (SEM) years; range, 34 to 67 years) 24 h after open-heart surgery (mitral valve replacement).

METHODS

Hemodynamic factors (pulmonary and radial artery indwelling catheters), oxygen and carbon dioxide partial pressures, oxygen saturation, oxygen delivery and consumption, and intrapulmonary shunt fraction were obtained before, during, and after lipid infusion (for 2 h), at 30-s intervals, along with some metabolic indexes (triglycerides, free fatty acids, glucose, insulin, lactate, acetoacetate).

RESULTS

No statistically significant changes in heart rate, cardiac index, systemic and pulmonary pressures and resistances, central venous and pulmonary capillary pressures, or arterial oxygen partial pressure were observed during infusion. Arterial carbon dioxide partial pressure values were constantly reduced throughout and after the end of lipid infusion, as compared with baseline values, while oxygen consumption was increased significantly without any change in oxygen delivery. No adverse effects on intrapulmonary shunt fraction were observed. Statistically significant increases of triglycerides, free fatty acids, acetoacetate and insulin (peak values at end of the lipid infusion) were found in comparison with baseline values. Plasma glucose increased significantly during lipid infusion and remained higher than baseline values until the end of the study. Lactate levels were unchanged except for a slight decrease at the end of the study, without any derangement of acid-base equilibrium. Neither arrhythmias nor adverse clinical reactions were observed as a consequence of lipid infusion.

CONCLUSIONS

Fat emulsions containing both MCT and LCT, when given at 3.3 mg/kg/min for 120 min following valvular heart surgery, do not exert negative cardiopulmonary effects, and could represent a source of rapidly metabolized substrates.

Artificial induction of intravascular lipolysis by lipid-heparin infusion leads to insulin resistance in man

Although extensive evidence indicates that free fatty acids can decrease glucose utilization in vitro, it is still controversial how an increase in lipolysis affects glucose metabolism in man. To test the hypothesis that an increase in lipolysis is related to insulin resistance, we examined the effect of lipid-heparin infusion on glucose metabolism in ten normal subjects by the euglycaemic glucose clamp technique and isotopic determination of glucose turnover. In the control euglycaemic clamp studies with insulin infusion at 0.2 and 1.0 mU.kg-1.min-1, endogenous glucose production was suppressed from the basal rate of 2.0 +/- 0.3 mg.kg-1min-1 to 1.1 +/- 0.7 mg.kg-1.min-1 and -0.4 +/- 0.7 mg.kg-1min-1 respectively. Glucose utilization increased from the basal rate of 2.0 +/- 0.3 mg.kg-1min-1 to 2.3 +/- 0.5 mg.kg-1min-1 and 5.9 +/- 1.8 mg.kg-1min-1 respectively. When the euglycaemic clamp studies were coupled with lipid-heparin infusion at comparable low and high rates of insulin infusion, endogenous glucose production increased (1.8 +/- 0.7 mg.kg-1.min-1, p less than 0.001, and 0.3 +/- 0.6 mg.kg-1.min-1, p less than 0.05, respectively), and glucose utilization decreased (2.1 +/- 0.3 mg.kg-1.min-1, not significant, and 3.2 +/- 0.7 mg.kg-1.min-1, p less than 0.001 respectively). These data suggest that the artificial induction of intravascular lipolysis by lipid-heparin infusion leads to a state of insulin resistance in man.

[Blood lipid and blood glucose changes during single, multiple and repeated stress]

As soon as various stressors exceed specific thresholds they induce marked metabolic changes in the organism. In a number of studies these facts have been verified experimentally. In our investigations we were able to demonstrate that the pattern of stress-induced changes differs after single and multiple stress exposures and that metoblic alterations in repeated stress situations are modified by habituative and adaptative processes respectively. Stress-induced metabolic mechanisms differently develop for blood lipid fractions and blood glucose. The described mechanisms are explained according to the functions of these blood parameters during stress exposures. The differential analysis of these alterations may be important for the prevention of stress-induced metabolic disorders.

Effects of sodium salicylate on plasma insulin concentration and fatty acid turnover in dogs

The effects of intravenous sodium salicylate administration on plasma concentrations of insulin, free fatty acids (FFA) and glucose were studied in intact, anaesthetized dogs both during basal and isoprenaline stimulated lipolysis. In both situations sodium salicylate reduced the plasma concentrations of insulin. The reduction was associated with decreased plasma FFA concentrations and FFA turnover rate, while plasma glucose concentrations remained unaltered. The reduced plasma insulin concentrations effected by sodium salicylate is most likely secondary to the concomitant fall in plasma FFA concentrations due to inhibition of FFA mobilization from adipose tissue.

Effect of heparin on insulin secretion in vivo and in vitro

The effect of heparin of therapeutic dose on the basal insulin secretion and on that of induced by glucose stimulation was studied in dogs and in Langerhans islets in vitro model. The effect of heparin, given intraarterially (a. pancreaticoduodenalis sup.) and injected i.v. before i.v. glucose load, on the immuno-reactive insulin concentration of the pancreatic and peripheral venous blood as well as the blood sugar and free fatty acid levels of the peripheral venous blood was investigated. Heparin inhibited significantly both the basal insulin secretion and that of following glucose stimulation. After the injection of heparin the glucose assimilation coefficient decreased significantly. The inhibitory effect of heparin on the basal insulin secretion and the glucose stimulated insulin secretion was observed in Langerhans islets in vitro model, too. This suggests that heparin, at least partly, inhibits insulin secretion directly through its influence on the beta cells.

Effect of lipids on insulin, growth hormone and exocrine pancreatic secretion in man

Influences of fat on release of insulin, growth hormone and pancreatic enzyme secretion were studied in 35 metabolically healthy subjects. A fat solution containing 40 g of soy bean oil was administered, I.V., orally and intraduodenally. In all cases there was a similar increase of insulin but the rise in serum insulin after oral or intraduodenal fat administration was not related to the changes in plasma free fatty acids, free glycerol and triglyceride levels. Blood surgar responded according to insulin secretion. The route of fat administration may possibly influence growth hormone secretion. Following intraduodenal fat administration volume and bicarbonate contents of the duodenal juice rose slightly whereas trypsin and bilirubin content increased considerably. These results suggest that insulin secretion after oral or intraduodenal administration of fat is influenced by intestinal factors. Cholecystokinin-pancroezymin and gastric inhibitory polypeptide are qualified to serve as such factors.

Stimulation of insulin secretion by infusion of free fatty acids

The acute elevation of plasma free fatty acid (FFA) levels by direct infusion of sodium oleate into the plasma of conscious dogs was accompanied by the rapid onset of a 2- to 12-fold increase in plasma immunoreactive insulin, and, subsequently, a marked fall in plasma glucose, even in dogs receiving intravenous glucose throughout the infusion. The magnitude of both the insulin and glucose responses correlated with the mean FFA level during infusion. A large increase in plasma insulin and fall in glucose also occurred when glycerol was infused with oleate in order to simulate endogenous lipolysis more closely. Insulin levels in pancreaticoduodenal vein blood rose markedly during oleate infusion, while plasma ketone levels rose only slightly. In contrast to the effects of oleate infusion, elevation of plasma FFA to correspondingly high levels by triolein ingestion and intravenous heparin produced only small increases in plasma insulin, which did not correlate well with the FFA level reached, and small increases in plasma glucose.The results indicate that under certain conditions elevated FFA levels may be a potent stimulus of insulin secretion. This response is modified under other conditions such as during chylomicron removal under the influence of heparin. This effect may play a role in the regulation of lipolysis and ketone formation, but determination of the exact mechanism of FFA stimulation of the pancreas and its physiological significance will require further investigation.

Infusion of long-chain fatty acid anions by continuous-flow centrifugation

We have developed a method for the rapid infusion into plasma of large amounts of long-chain free fatty acids (FFA). Unanesthetized dogs were connected by a peripheral artery to a closed, continuousflow centrifuge from which cells and plasma emerged in separate lines. Sodium oleate was infused directly into the plasma line before cells and plasma were recombined and returned to the animal through a peripheral vein.The centrifugation procedure itself produced only small changes in circulating levels of glucose, FFA, and electrolytes. Plasma flow rates as high as 100 ml/min could be maintained, and centrifugations of 12 hr were accomplished without complications. During centrifugation, sodium oleate was infused at rates up to 80 muEq/kg per min for 2.5 hr; the maximum molar ratio of FFA to albumin without hemolysis was 10:1. Plasma FFA levels rose rapidly after infusions were started and reached constant elevated levels within 15-20 min. Oleate infusion at 10-50 muEq/kg per min produced a rise in plasma FFA proportional to the infusion rate. The maximum increment in plasma FFA above control values was 1.66 muEq/ml. When infusions ended, plasma FFA declined rapidly to control levels. Oleate infusion at rates below 30 muEq/kg per min did not reduce levels of other plasma FFA. Infusion at high rates was accompanied by a marked fall in blood glucose. This method permits adminsitration of long-chain fatty acids in sufficient quantities to study their individual metabolic effects, and provides a new way to supply lipid calories parenterally.